The present invention is related to the field of spread spectrum communication systems and more particularly, to a spread spectrum receiver which utilizes a detector that incorporates a dither detection technique.
Spread spectrum communication systems utilize a transmission bandwidth which is many times as large as the information bandwidth in order to achieve jam resistance.
Additional advantages of such systems include multipath signal rejection, and a low probability of unauthorized detection. In a spread spectrum communication system, a spread spectrum transmitter generates a date modulated signal, which modulated signal has its energy spread over a frequency band that is considerably wider than the frequency band of the data rate. Generally, this is accomplished with a carrier signal that is first modulated by an information or data signal, of relatively narrow bandwidth, and then by a wide bandwidth signal such as a pseudo-random (PR) signal or a pseudo-noise (PN) signal. The cumulative effect of this double modulation is to widen the bandwidth of the total signal to that of the widest modulating signal.
Within the receiver, the received spread spectrum signal is correlated against a signal corresponding to the modulating PN sequence signal utilized in the transmitter. The effect of this correlation, is to spread any received CW (continuous wave) type jamming signal over a wide band of frequencies so as to enable a filter to remove the major energy portions of the jamming signal from the system.
One of the difficulties encountered with synchronizing a local PN sequence, generated in the receiver, to the PN sequence component incorporated in the received modulated signal (generated in the transmitter) is that when the local PN sequence is locked to the receive PN sequence, there usually exists a band in which locking is indicated. Therefore, although a locked condition is indicated, it may be that the match between the two sequences is "off" (out of phase) by an amount corresponding to the range, or width, of the band within which lock is indicated. Utilization of a dithering technique allows the band, within which a lock indication is provided, to be decreased to a smaller amount, which thereby decreases the deviation between the local generated PN sequence and the received PN sequence component in the received signal. In those receivers where both the local dithered PN sequence and the received data signal pass through the same PN correlator, the amount of PN dither has to be held to a small percentage of a PN baud period, otherwise the data error may be too large under low signal-to-noise conditions. A baud period, for information purposes, corresponds to a time segment of a digital sequence. Since the amount of dither has to be held to a small percentage, the PN loop performance may deteriorate to the point where, under low signal-to-noise conditions, proper phase locking is not achieved.
One prior art patent of particular interest is U.S. Pat. No. 3,706,933 entitled, Synchronizing System in the Presence of Noise, by F. W. Bidell et al. The system disclosed in the referenced patent utilizes a locally generated coded signal and an orthogonally related version of the locally coded signal as the demodulation signals. The received signal is directed to a signal channel wherein a first mixer hetrodynes the locally coded signal with the received signal and wherein the received signal is also directed to a reference channel wherein a second mixer hetrodynes the orthogonally related verion of the locally coded signal with the received signal. The demodulated signals from the mixers are then compared and utilized to drive the locally coded signal into synchronization with the received signal.
Additional patents of interest for their teachings are:
U.S. Pat. No. 3,350,644 entitled, "Slip Corrector Means and Method for Multistation Networks", by R. J. McNair;
U.S. Pat. No. 3,351,859, entitled "Communication System Employing Multipath Rejection Means" by E. J. Groth, Jr. et al.;
U.S. Pat. No. 3,478,268, entitled, "Suppression of Strong Interfering Signals in a Radio Receiver", by G. J. Coviello;
U.S. Pat. No. 3,665,472, entitled, "Two-Way Communication System Employing Two-Clock Frequency Pseudo-Noise Signal Modulation", by Kartchner et al.;
U.S. Pat. No. 3,699,447, entitled, "Wideband Frequency Modulation Communications System," by W. T. Frost;
U.S. Pat. No. 3,714,573, entitled "Spread-Spectrum Position Monitoring System", by B. C. Grossman;
U.S. Pat. No. 3,731,198, entitled "Synchronization Device for Anit-Jamming Communications System," by H. L. Blasbalg;
U.S. Pat. No. 3,878,334, entitled, "Data Synchronizing Systems", by P. H. Halpern; and
U.S. Pat. No. 3,878,468, entitled, "Joint Equalization and Carrier Recovery Adaptation in Data Transmission Systems", by D. D. Falconer, et al.
The present invention directs itself to the particular problem of providing an increased, or improved, signal-to-noise ratio in the PN loop of a spread spectrum receiver without degrading the data error performance.